EP0223667A1 - Process for marine seismic prospection using a coded vibratory signal, and device for carrying out such a method - Google Patents

Abstract

Method of marine prospecting in seismic-reflection using a vibratory signal coded according to a pseudo-random code. The correlation of the signal received on the seismic sensors by the cycle of the coded signal emitted makes it possible to obtain seismic traces analogous to what would make it possible to obtain a pulse source firing at time intervals equal to the duration of the cycle of the coded signal. The correlation with offset cycles makes it possible to reduce the distance between traces recorded on the same sensor. Emissions from two or more vibrators, emitting the same coded signal with cycles used.

Description

The present invention relates to a method of marine seismic prospecting using one or more vibrators emitting signals with phase variation.

Terrestrial seismic prospecting methods are known comprising the transmission in the ground for several seconds of a vibratory signal whose frequency varies continuously within a frequency band, the reception by sensors of the signals reflected by underground reflectors and recording of received signals. Due to the duration of the emission, the signals picked up at each instant are combinations of signals reflected by reflectors located at very different depths. The image of the different reflectors in the basement can only be found by processing the signals received, including their correlation with the signals transmitted. The result of the processing is identical to that obtained by convolving the autocorrelation function of the signal emitted as a result of the reflection coefficients of the different reflectors. A seismic trace is obtained, which is the image of the interfaces between the different geological layers halfway between the transmission and reception locations.

Such a process is described, for example, in US Patent N, 2,688,124.

This process has certain drawbacks. The autocorrelation function which is obtained in this case has secondary peaks on either side of the main peak, the amplitude of which is not negligible. In addition, a time interval at least equal to the duration of the outward and return propagation of the waves emitted to the deepest reflector of the explored area which is designated by "listening interval" must be made between two sequences. successive transmission signals, so that the strong signals picked up at the start of the corresponding recording sequence cannot mask the weakest signals emanating from more distant reflectors, picked up at the end of the previous recording sequence. The necessary interruptions of the transmission during a relatively long listening time interval have the effect of limiting the transmitted energy. They are particularly troublesome in marine seismic prospecting where, due to the continuous advancement of the ship towing the transmission-reception assembly, the difference between the successive positions where the sequences occur is significant, which limits the spatial resolution recordings.

Another method is also known where, to suppress the listening time intervals, two signals obtained by a linear scan of the same frequency band are alternated, one in the increasing direction, the other in the direction decreasing. It allows continuous transmission and recording but has the disadvantage of creating significant correlation "noise". Such a method is described in US Pat. No. 3,413,596.

Other known methods of terrestrial seismic prospecting include the use of vibratory sources transmitting signals obtained by modulating a carrier signal by a binary signal or pseudo-random code consisting of a sequence of elements which can take two logical values 0 or 1. The order of succession of these values is chosen so as to be random. A logical "1" leaves the sinusoidal signal unchanged. A logical "0" reverses the phase.

The code is of the maximum length type, that is to say that any sequence of n successive bits can be repeated identically only after a sequence of (2 ⁿ -1) bits. The transmitted signal consists of a series of identical cycles, the duration of which is fixed by the frequency of the carrier signal and by the number of terms of the chosen code. The duration of the transmitted signal is chosen long enough to increase the transmitted energy.

Such methods are described, for example, in US patents N, 3,234,504 or 3,264,606.

The previous methods are suitable for earthquake prospecting where the vibration source is moved discontinuously from one point to another in a succession of locations of a plane of seismic profile to be studied.

The seismic source being fixed during each emission or emission sequence, as well as the locations of the underground reflectors returning the energy which is captured by the receivers, theoretically no limit is imposed on the duration of emission.

The method according to the invention is suitable for prospecting, marine seismic where the transmission means and the receivers are towed by a ship progressing continuously along a seismic profile to be studied.

It comprises the emission by at least one vibrating source towed from repetitive sequences of acoustic vibrations modulated in a non-repetitive manner during each of the sequences, the reception of the acoustic waves returned by the submerged reflectors, the recording acoustic signals received and their processing to determine the position of the reflectors.

It is characterized in that the transmission comprises a series of chained sequences each consisting of a periodic carrier signal modulated in phase by a pseudo-random binary coding signal of maximum length, and in that the processing of the acoustic signals received comprises correlating the signals received during a time interval equal to the duration of several successive sequences with the coded signals transmitted, so as to obtain correlation peaks at time intervals less than or at most equal to the repetition period of the sequences d successive transmission, which allows, by cutting the composite signal resulting from the correlation, into parts of the same length as each sequence, to reduce to traces corresponding to regularly distributed isolated shots, to which conventional treatments of multiple coverage.

According to a first embodiment, the vibration source is unique, the correlation is carried out between the received signals and alternately a sequence of transmitted signals and the same sequence of signals shifted in time by an interval less than the period of repetition of the sequences . For example, an offset is chosen equal to the half repetition period of the transmitted signal sequences.

According to another embodiment, at least two vibrating sources emitting are used simultaneously, the sequence of the sequences of signals emitted by one of the sources being the same as that emitted by the other but shifted in time and a correlation is established. between the signals received resulting from the signals emitted simultaneously by the sources and the sequence of coded signals emitted by one or the other of the two vibration sources, so as to alternately obtain correlation peaks corresponding to one or the other vibrational sources.

The method according to the invention is advantageous in that:
the correlation noise is particularly low, the amplitude ratio between each main correlation peak and its secondary peaks being equal to the number of code terms and this for practically the entire duration of the composite signal resulting from the correlation, except for one duration equal to the length of the sequence which is negligible taking into account the fairly long duration of the received signals selected to obtain the composite signal;
- transmission and recording are carried out without interruption. It follows that the energy transmission is optimal and that the time interval between the successive recording traces can be reduced to the listening time;
- the shape of the correlation peaks which plays the same role as the signature of an impulse source, is known and repetitive, which allows a better effectiveness of certain treatments, in particular of multiple coverage;
- The emission spectrum is better suited than the linear frequency scanning of previous processes, to the possibilities of transmission of marine vibrators, generally ineffective at low frequencies;
- the cyclic nature of the pseudo-random codes and their permutation possibilities which will be explained later, allow, by correlations between the recordings and shifted sequences, to obtain intermediate traces and thus improve the spatial resolution of the restored seismic profile, the intertrace can be reduced to less than half the distance traveled by the ship during the listening time; and - the use of at least two sources simultaneously emitting signals obtained with offset codes, allows the records corresponding to each of them to be separated.

• - la figure 1 représente schématiquement un ensemble d'émission-­réception sismique remorqué par un navire ;
• - la figure 2 représente schématiquement un dispositif pour engendrer un code pseudo-aléatoire ;
• - la figure 3 montre un exemple d'un signal de codage pseudo-aléatoire ;
• - la figure 4 montre un signal sinusoïdal porteur modulé en phase par le signal de codage de la figure 1 ;
• - les figures 5 et 6 représentent respectivement, de façon schématique, une séquence de signal modulé et la fonction de corrélation obtenue lorsque des interruptions sont ménagées entre les cycles d'émission successifs ;
• - les figures 7 et 8 représentent respectivement deux séquences d'émission décalées l'une par rapport à l'autre d'une demi-période, utilisées pour la corrélation avec les signaux reçus ;
• - les figures 9 et 10 représentent respectivement la disposition des pics de corrélation principaux obtenus en utilisant les deux séquences décalées ;
• - la figure 11 représente des signaux captés en réponse à l'émission simultanée par deux sources de séquences de signaux décalées les unes par rapport aux autres ;
• - la figure 12 représente une séquence de signaux émis avec laquelle les signaux reçus représentés à la figure 10, sont corrélés ;
• - la figure 13 représente la succession de pics de corrélation résultant de cette corrélation ; et
• - la figure 14 représente le schéma synoptique d'un dispositif pour la mise en oeuvre du procédé.

Other characteristics and advantages of the method and those of an embodiment of a device for its implementation will appear on reading the description below, with reference to the appended drawings where:

• - Figure 1 schematically shows a seismic transmission-reception assembly towed by a ship;
• - Figure 2 schematically shows a device for generating a pseudo-random code;
• - Figure 3 shows an example of a pseudo-random coding signal;
• - Figure 4 shows a carrier sinusoidal signal modulated in phase by the coding signal of Figure 1;
• - Figures 5 and 6 show, schematically, respectively, a modulated signal sequence and the correlation function obtained when interruptions are made between successive transmission cycles;
• - Figures 7 and 8 respectively show two transmission sequences offset from each other by a half-period, used for correlation with the received signals;
• - Figures 9 and 10 respectively show the arrangement of the main correlation peaks obtained using the two offset sequences;
• - Figure 11 shows signals picked up in response to the simultaneous transmission by two sources of signal sequences offset from each other;
• - Figure 12 shows a sequence of transmitted signals with which the received signals shown in Figure 10, are correlated;
• - Figure 13 shows the succession of correlation peaks resulting from this correlation; and
• - Figure 14 shows the block diagram of a device for implementing the method.

The transmission-reception device comprises (Fig. 1) one or more vibration sources 1 of a known type, hydraulic vibrators for example, towed underwater by a ship 2 at the end of a power cable 3 Each source can include several transducers vibrating in phase. It also includes a receiving assembly such as a seismic flute, also towed underwater. Source 1 is supplied by a signal consisting of a series of identical transmission cycles, each of them consisting of a coded synusoidal signal. The coding signals used for the modulation are preferably binary sequences of maximum length.

A binary sequence of maximum length is a set of (2 ⁿ -1) binary "words" that can be formed from n bits. These binary words are generated using (Fig. 2) linear shift registers (LSR) 5 with n bits which can each take two logical states 1 or 0. In the example of Figure 2 where n is equal to 3, a summator 6 adds up the two most significant bits and applies it to the input of register 5, causing an offset of its content. The register being initialized to any value other than zero, 101 for example, we see that by successive summations and shifts, it displays 7 distinct words, this sequence reproducing identically continuing the same process. The least significant bit of the register will successively display all the binary values of the sequence 1110010 etc.

Maximum length binary sequences have the following properties:
- the number 1 of the binary words is approximately equal to the number of 0;
- the division of the binary words into segments containing bits of identical value, shows that half of them contains only one element, the quarter contains 2, the eighth contains 3, etc. ;
- the auto-correlation function of binary words presents a peak at the origin and decreases very quickly beyond.

These properties are very close to those of a purely random sequence.

Such a so-called pseudo-random sequence, a particular example of which is shown in FIG. 3, is used to code a sinusoidal signal (FIG. 4). The phase of the signal is reversed each time one passes from a binary value to that which follows it in the coding sequence. The inversion takes place at successive instants when the elongation of the vibratory movement is zero so that the vibratory source can follow the imposed movement and therefore each bit of the sequence must encode a multiple of the half-period of the sinusoidal signal.

It is shown, and this is particularly important, that the ratio of the amplitude of each main peak of the correlation function to that of the secondary peaks which surround it is equal to the number of elements of the coding sequence.

The coding sequence formed from an 8-bit register will have 511 elements. If such a sequence is used to modulate a signal carrying 51Hz, the duration of each vibratory cycle will be 10 seconds and the ratio of the amplitude of each main peak to that of the corresponding secondary peaks will be equal to 54dB.

With such a ratio, the main correlation peaks associated with the weak reflected signals received at the end of a recording cycle are not masked by the secondary peaks associated with the strong signals received at the start of the next cycle, if the depth of investigation is not too great. Transmission and recording can be performed continuously.

In Figure 5 is shown schematically a carrier sinusoidal signal modulated by a pseudo-random code of 31 terms. The ratio of the amplitude of a main peak P₁ of the corresponding correlation function (Fig. 6), to the secondary peaks P₂ (correlation noise) is equal to 31 when the emission takes place without interruption.

We see in part B of Figure 6 the increase in correlation noise caused by an interruption of transmission and recording for the duration of a cycle and therefore the degradation of the amplitude ratio of each peak main to the resulting correlation "noise". This example justifies the interest of the uninterrupted transmission and recording made possible due to both the continuous progress of the ship and the transmission-reception unit along the profile, and the use pseudo-random coding.

According to one embodiment of the method, a correlation of the received signals is established with two sequences of transmitted signals of duration T (Figs. 7, 8), which are deducible from one another by a time offset. We use the properties of the following pseudo-random codes which two sequences formed from the same code, by performing a circular permutation in the order of its terms, have the same properties. In the example shown, the two sequences are coded by codes with 31 terms offset from each other by 15 terms. We see that the correlation function obtained with the first sequence (Fig. 9) has main peaks whose repetition period is T. The one obtained with the other sequence (Fig. 10) also has a series of successive main peaks with the period T. But the two series obtained are offset with respect to each other by a time interval which depends on the difference between the two coded sequences used. The resulting series is shown in Figure 10. In the example shown, the difference is equal to T / 2. Although the information contained in the two intermediate peaks is not independent of that contained in the two neighboring peaks of the resulting series, the duplication of the peaks makes it possible to obtain results similar to the mixing operations carried out in a conventional manner by overlapping the successive positions of the transmission-reception device.

The example above, where the offset between the signal sequences chosen to perform the correlation is equal to the half-period T / 2, is not limiting. More generally, the offset is chosen so that the time interval between peaks of the resulting series is less than or equal to the "listening" duration, that is to say the interval of maximum propagation time of the acoustic waves throughout the area explored.

According to another embodiment, two vibration sources are used towed by the same ship and offset laterally on either side of its trajectory. The two sources are fed simultaneously by two coded vibratory signals obtained by modulating the same sinusoidal signal by two pseudo-random sequences which are deduced from each other by a time offset or a circular permutation of their terms. The resulting signal (Fig. 11) received by the reception assembly is the sum of two coded signals analogous to the signal shown in FIG. 12 and shifted for example by half a period T.

A correlation is established between the resulting signal and the sequence transmitted by one of the two sources. We obtain a resulting series of correlation peaks whose spacing is still equal to T / 2 (Fig. 13) which alternately contain information associated with one of the sources and the other. If the duration of the vibration cycle is at least equal to twice the listening time, it is thus possible to separate, during the correlation step, the data associated respectively with the two sources, although they operate simultaneously.

The space between the traces corresponds to the progress of the ship during the duration of the vibratory cycle but here too it is possible to reintroduce intermediate traces on each of the seismic profiles restored by correlating the recordings by the sequence emitted alternately by one and the other vibrators.

The process can be generalized to several vibrators and find applications in the field of broadband seismic prospecting where the ship tows two vibrators offset laterally by a few tens of meters, on either side of the seismic recording flute, or even in the field of three-dimensional seismic prospecting.

The device for implementing the method is associated with a central control and recording system 5 adapted to sequentially collect all the seismic data collected by the seismic streamer (FIG. 1) and comprise a control assembly 6 and a set of recording 7 comprising at least two tape recorders 8, 9 adapted to record alternately the multiplexed data transmitted to the control unit. Such a system is described for example in French patent N, 2,471,088 relating to a multiplexed seismic flute. The device also comprises a pseudo-random signal generator 10 which delivers its signals, on the one hand, to a control member 11 of the vibration source 1 and, on the other hand, to the recording unit 7. Each recorder 8 or 9 records the signals transmitted and received during a series m of successive transmission-reception cycles. The alternation of the recordings is carried out with an overlap of a cycle, that is to say that the last cycle of a series is recorded by the two recorders.

The recorded data will be applied in a subsequent step to a computer 12 adapted to reconstruct the seismic traces obtained along the profile explored and then are correlated with the pseudo-random sequences transmitted, in accordance with the method according to the invention. Correlations are carried out by the computer suitably programmed for this purpose.

Claims (9)

1. - A method of marine seismic prospecting comprising the emission by at least one vibration source towed by a vessel moving continuously in a profile plane to be studied, of repetitive sequences of acoustic vibrations modulated in a non-repetitive manner during each of said sequences , the reception of the acoustic waves returned by the reflectors, the recording of the acoustic signals received and their processing to determine the position of the said reflectors, characterized in that the emission comprises a sequence of linked sequences each consisting of a modulated periodic carrier signal in phase with a pseudo-random binary coding signal, and in that the processing of the acoustic signals received comprises the correlation of the signals received for a duration equal to that of several successive sequences with a single sequence of coded signals, so as to obtain correlation peaks at time intervals less than or equal to the period of repetition of the successive emission sequences. 2. - Method according to claim 1, characterized in that a single vibration source is used, the correlation being carried out between the signals received and alternately the sequence of signals transmitted and the same sequence shifted in time by interval lower than the repetition period of the transmitted sequence. 3. - Method according to claim 1, characterized in that at least two vibrating sources emitting are used simultaneously, the sequence of signal sequences emitted by one of the sources being the same as that emitted by the other, but shifted in time and in that one establishes a correlation between the signals received resulting from the simultaneous emissions of the two sources, and the sequence of coded signals emitted by one or the other of the two vibration sources, so as to obtain alternating correlation peaks corresponding to the first and second vibration sources. 4. - Method according to claim 2, characterized in that the two sequences used for the correlation are offset with respect to each other by a time interval less than the maximum duration of propagation of the acoustic waves in the area to explore. 5. - Method according to claim 2, characterized in that the time difference between the two sequences used for the correlation is equal to the half-period of repetition of the transmitted signal sequences. 6. - Method according to claim 1, characterized in that at least two vibrating sources emitting are used simultaneously, the sequence of sequences of signals emitted by one of the sources being the same as that emitted by the other, but shifted in time and in that one establishes a correlation between the signals received resulting from the simultaneous emissions of the two sources, and alternatively, the sequence of coded signals emitted by one or the other of the two vibration sources, so to obtain alternating correlation peaks corresponding to the first and to the second vibration sources. 7. - Device for implementing the method according to claim 1, characterized in that it comprises means for generating vibrations in water, a central system (5) for controlling and recording the collected seismic data by a reception device (4) of the multiplex transmission type, a coded signal generator (10), the signals of which are applied to the means for generating vibrations (5) and processing means (12) for establishing correlations between the signals generated by the coded signal generator and received signals recorded by the central system. 8. - Device according to claim 7, characterized in that the central control and recording system (5) comprises two recorders (8, 9) which each record the received signals corresponding to a predetermined number of sequences, a fraction of the received signals being recorded at the same time on the two recorders. 9. - Device according to claim 7, characterized in that the means for generating vibrations comprise two vibrators offset laterally with respect to the trajectory of the ship.

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